1. Field of the Invention
The present invention relates to a charging roller, which is applicable to electrical charging in an electrophotographic image-forming apparatus, and which is used in contact with a chargeable member (or a charge-receiving member) to charge it electrically by application of voltage. The present invention also relates to a process cartridge and an image-forming apparatus employing the charging roller.
2. Related Background Art
Conventionally, for electrophotographic image-forming apparatuses, a corona charger is used as the primary charging device for the image-holding photosensitive member, which applies a high voltage to a wire to cause corona discharge toward the chargeable member. In recent years, a contact type of charger is being developed which is used in direct contact with the chargeable member to charge the surface of the chargeable member by application of voltage. The contact charging is advantageous in that the required voltage is low for obtaining a necessary surface potential of the chargeable member and ozone is generated less at the charging process in comparison with the non-contact corona charging. In particular, roller charging with an electroconductive roller as the charging member is widely used owing to stability of the electrical charging.
FIG. 12 illustrates an example of a charging roller employed in a conventional electrophotographic image-forming apparatus.
A charging roller 110 is constituted of an electroconductive axis 111 serving also as a power supplying electrode, an elastic layer 112, and a coating layer 113. Conventionally, the elastic layer 112 is made from a solid rubber such as styrene-butadiene rubbers (SBR), isoprene rubbers, and silicone rubbers, and the coating layer 113 is made from a resin or a rubber such as polyamide resins, hydrin rubbers, urethane rubbers, and silicone rubbers. The charging roller is liable to generate noises in combination with the photosensitive member on application of an AC bias. In order to reduce noise, various efforts have been made such as filling empty spaces of the photosensitive member with weights, use of a sponge material as an elastic layer, and use of a resin tube as a coating layer.
However, the above-described conventional charging roller has a high hardness, and the one employing a sponge has a rough surface. Therefore, the boundary of the discharge region on the charged photosensitive member is not sufficiently linear in the length direction, as that shown in FIG. 4 where the numeral 42 indicates a discharge region, and the numeral 41 indicates the region opposing the nip portion between a photosensitive member and a charging roller. Such a non-uniform discharge tends to cause non-uniform wearing of the surface of the photosensitive member during repeated use, resulting in shortening of the life of the photosensitive member as the result of wear. In a high processing speed, in particular, a higher frequency of an AC bias is applied (1000 Hz or more) to allow a large electric discharge current to flow between the photosensitive member and the charging roller, which damages the photosensitive member more greatly to render the above-noted disadvantage more serious.